A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Along the elongate axis of the electrode array 110 on its surface are multiple electrode contacts 112 that provide selective stimulation of the cochlea 104.
The standard stimulation pulses in cochlear implants are biphasic. As shown in FIG. 2, such biphasic stimulation pulses have a negative half-wave (cathodic phase) and a charge-balanced positive half-wave (anodic phase). The net charge of a given half-wave pulse corresponds to the product of its current amplitude A and its pulse duration T. To ensure that no DC components are transmitted to the auditory nerve, the biphasic stimulation pulse includes an opposite phase half-wave pulse of equal duration and opposite amplitude to the first half-wave pulse. In specific pulsatile stimulation strategies, sequential or parallel pulses can be generated at different stimulation electrodes.
Sometimes, depending on the propagation of the electrical stimulation field and the specific anatomical situation, other nerves may be stimulated inadvertently. Such collateral stimulation can result in unintended side-effects such as twitching of the eye or a burning sensation in the tongue or in the throat. These unpleasant side-effects increase in intensity with increasing charge. In some cases, this situation can prevent setting the stimulation intensity sufficiently high for effective hearing via the cochlear implant. If only one or a few stimulation electrodes are affected, these stimulation electrodes can be deactivated. But this change in the operation of the cochlear implant may have other undesirable consequences for the patient. If a considerable number of or all of the stimulation electrodes are affected, the cochlear implant may not be usable for hearing in extreme cases.
When setting the stimulation parameters in a patient fitting process, the fitting audiologist can try to change various stimulation parameters such as pulse width, stimulation rate and compression to provide a louder auditory sensation and reduce the side-effects. Re-implantation with a cochlear implant with a differently arranged reference electrode also has been attempted by placing separate ground electrodes at very specific locations. EP 0959 943 mentions that facial nerve stimulation is an unwanted somatic effect. US 2012/0143284 also discusses the problem of undesirable facial nerve stimulation and other unwanted side-effects. But throughout these documents this issue is always discussed in connection with extra-cochlear electrodes which are considered the source of these effects.
U.S. Pat. No. 5,601,617 describes selecting complex stimulus waveforms including triphasic pulses based on the “response” of the stimulated tissue. Generally the discussion assumes that this is the perceptive response and there is no mention of mitigating adverse stimulus effects.